Considering that players who previously strengthened Napoli also left, and there was rumours that Napoli would sell other key players in the transfer market, this made Spalletti decision more unanimous to leave Napoli. As you said Leaving after winning that trophy yes is far better, because indeed him service that will be remembered  by all people especially their fans rather than leaving when don't get any trophy, although there is still a possibility they will win again but maybe Spalletti doubts that.